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The one I probably use the most is Show Anchors, which I made. It shows an anchor icon next to every element on the page that can be linked to directly.
Sounds stupid, but it's very useful when you want to refer someone to a specific section of a website. And it's much easier than trying to figure out where the site owner put their anchor links (if they ...

I use StackPrinter bookmarklet; it allows to print Stack Exchange questions (like this one) in a printer-friendly way.
javascript:(function(){var re = new RegExp('^http://(.*?)\.com/questions/([0-9]+)/');var group = re.exec(window.location.href);if (group!=null){var service = group[1];var ...

Quix: http://quixapp.com
This is a little bit like Quicksilver for the web. It's very useful for web development. You can also configure Chrome to use it as a search engine to turn the address bar into a command line.

Peter Steinberger has developed a Chrome and Safari extension called "stackoverflowerizer" that automatically redirects pages on efreedom.com, questionhub.com, etc. to the original StackOverflow page. It will rewrite your Google search results (to link directly to SO), but it won't remove the spam sites from Google.

Here's my own bookmarklet for all Stack Exchange sites:
This code will show your upvote count for all tags that appear in your user page's top tags section. It should work on any Stack Exchange site you are logged in on (but I've only tested it on Stack Overflow and Meta Stack Overflow). Unfortunately there is a bug in the tag synonym redirection logic that ...

If all you want to do is to prevent yourself from accessing certain domains there are a few options that immediately jump to mind. None of these will stop google returning these sites as results, but you won't be able to access them when it does.
Edit your HOSTS file.
Configure your router (if your router supports blocking of domains).
Use a DNS service ...

You can try this GreaseMonkey script. I haven't tried it myself, but it was recently updated, so it should updated a year ago, but it will still work with Google's current results page, although instant search needs to be off. Simply edit the script and add the unwanted sites to the list of blocked sites.

If your application uses the HTTP protocol, then you may use the Charles Web Debugging Proxy:
Charles is an HTTP proxy / HTTP monitor / Reverse Proxy that enables a developer to view all of the HTTP and SSL / HTTPS traffic between their machine and the Internet. This includes requests, responses and the HTTP headers (which contain the cookies and caching ...

Apart from a user script (which most likely would be what be needed to interrupt the scroll), it takes me two quick scrolls to get to the bottom.
This could also be accomplished with End the key (on Mac Cmd and ↓ )
It is not a nice solution if you are not good quick scrolls, but it does the job for me.
So far all the user-scripts I have seen have tried to ...

There is no "UnfriendFinder" alternative at the moment.
However, you can export your friendlist to a .xls or .csv file. If you want to check, you just have to export your friendlist another time and compare it with the first one.
This solution is very basic but it does work well.

I haven't found any extension like what you're describing either.
A workaround could be to use Adblock Plus or another similar tool and block the specified elements on the sites. You have to do it on every site but I guess you'll eventually have saved time on it.

If you use GreaseMonkey, you can hide the Hot Network Questions.
CleanTube would work for YouTube.
I agree with Punchlinern's sentiment, that instead of trying to find one add-on that will work across sites, you are looking at a bunch of workarounds rather than a tool that is smart enough to recognize what's central to a page vs. a distraction. Keep ...

The keyboard shortcut for opening the diigolet sidebar appears to be disabled by default, so you if you open about:config and search for "extensions.diigotb.keys.sidebar"
set enabled to true;
extensions.diigotb.keys.sidebar.enabled = true
then control-alt-s can be used to open and close the diigolet sidebar
extensions.diigotb.keys.sidebar.keys = ...

Place an entry in your hosts† file that associates the efreedom.com domain names with the ip-addresses of (say) stackoverflow.com.
This won't affect Google's results but clicking them will take you to wherever you have configured.
† e.g. C:\WINDOWS\system32\drivers\etc\hosts on Windows XP.